High voltage MOSFETs are a type of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) that is designed to operate with high voltage differentials presented across terminals of the transistor. High voltage differentials necessarily results in a formation of electric fields within the device. High voltage differentials can be tolerated when the attendant electric fields nowhere exceed a critical threshold that causes avalanche breakdown. Electric fields can be decreased by distributing the field-generating high voltage differentials over large spatial dimensions of the semiconductor device.
Low conductivity regions of a semiconductor device, if arranged in a proper manner, can be used to distribute high voltage differentials. Such low conductivity semiconductor regions can include regions that are substantially depleted of majority carriers, regions that are lightly doped, and regions that are undoped. Such high voltage differentials should be distributed both laterally and vertically to ensure that attendant electric fields do not exceed the critical threshold that causes avalanche breakdown.
Therefore, high voltage MOSFETS can be manufactured using thick epitaxial layers that are lightly doped. But such thick lightly-doped epitaxial layers are not compatible with low voltage high-density CMOS devices. Low voltage high-density CMOS devices typically are manufactured on wafers that have relatively thin epitaxial layers that are more heavily doped than can be tolerated by high voltage devices. Various circuit applications require both dense logic, which can be manufactured using low voltage high-density CMOS devices, and high voltage transistors, which require generous spatial distributions of high voltage differentials.